In a line of light projected onto a linear spatial light modulator in an optical printing apparatus, onto a wafer in semiconductor process, or the like, it is usually preferred that light intensity along the line is as uniform as possible. This is because it is desirable that variations in uniformity of light projected from the spatial light modulator, for example, to a screen or a recording medium or to the wafer, are only the wanted variations representing information content of the projected light.
A diode-laser bar or a stack of such diode-laser bars is preferred as a light source in line-of light projecting apparatus because it is both compact and electrically efficient. A diode laser bar has a length of about 10 millimeters (mm) and a width of about 1.0 mm. The bar includes a plurality of spaced-apart individual diode-lasers (emitters) having a length in the width direction of the bar and a width in the length direction of the bar. The aperture of an emitter has a height (perpendicular to the length and width of the bar) of about 1.0 micrometer (μm). The width of the emitter is usually selected based on some compromise between a required power per emitter (in general, the wider the emitter the higher the power) and the quality of the beam emitted by the emitter (in general the narrower the emitter the higher the quality).
Another consideration in selecting emitter width is a parameter referred to as the fill-factor of a diode-laser bar. This is the fraction of the length of the bar that is occupied by the total width of emitting apertures. As there must be a certain minimum space between emitters, for example, between about 10 μm and 20 μm, clearly, the wider an individual emitter the less are required to occupy the length of the bar, the less is the length of the bar that is occupied by space between emitters, and the higher the possible fill factor. In a high power, for example, about 100 Watts (W) total, diode-laser bar, a common emitter width is about 100 μm. This allows a fill factor as high as 90%.
The height direction of the bar (and emitters) and the width direction of the bar (and emitters) are generally referred to as the fast and slow-axes of the bar (and emitters), respectively. The near-field emitted uniformity of a 100 μm emitter in the slow-axis (width direction) can vary by as much as between about 10% and 30%, here, defined as the difference between the maximum and minimum intensity divided by the maximum intensity. Prior-art line-of-light projectors usually function by projecting the height of the emitters as the height or width of the projected line of light and projecting the width of the emitters as the length of the line of light. It is usual to provide an optical system that sums the light in the width direction of the emitters while homogenizing light in the slow-axis using one or a plurality of slow-axis cylindrical microlenses in some configuration.
Diode-laser-bar-illuminated line-of-light projecting apparatuses functioning generally in this manner, but differing in implementation, are described in U.S. Pat. No. (5,517,359) to Gelbart, U.S. Pat. No. 6,494,371 to Rekow et al., U.S. Pat. No. 6,773,142 to Rekow, and U.S. patent application Ser. No. 10/667,675 filed Sep. 22, 2003, which is assigned to the assignee of the present invention. Each disclosed apparatus has a property that light intensity distribution changes due to failure of an individual emitter are minimized. It is believed, however, that slow-axis light-intensity variation in any one of these apparatus will be about 5% at best. There is a need for a diode-laser-bar-illuminated line-of-light projecting apparatus capable of projecting a line having a slow-axis light intensity variation less than prior-art apparatus while still retaining the insensitivity to failure of an individual emitter.